It is common in gas turbine engines for there to be adjacent regions containing gases in which the gas in one region is at a higher pressure than that in the other region. Typically a seal is provided between the regions in order to ensure that the leakage of gas from the higher pressure region into the lower pressure region is minimised. The provision of such a seal between components that are relatively movable gives rise to increased difficulties. Although seals are known that rely on physical contact with both components, relative velocities of adjacent components in gas turbine engines can be such that seals of this type are subject to rapid wear.
An alternative seal design that has been employed in gas turbine engines where rapid relative rotary motion between components takes place is known as a hydraulic seal. Typically such a seal is defined between concentric shafts and is constituted by a radially inner component and a radially outer component. The radially outer component is attached to the radially outer shaft and is of annular U-shaped cross-section configuration; the open part of the U-shape being radially inward of the remainder thereof. The radially inner part is attached to the radially inner shaft and comprises at least one radially outwardly extending annular fin. The fin is of such a size that the major part of it locates, in non-contacting relationship, within the radially outer component.
In operation, when the radially outer seal component is rotating at a sufficiently high velocity, oil is directed into it. The oil is centrifugally retained within the radially outer component to define an annular oil reservoir into which the fin of the inner seal component extends. A gas seal is thereby defined. The oil flow rate is chosen such that at low rotational speeds during engine starting, there is sufficient oil present to ensure the operation of the gas seal. When normal engine operating speeds are reached, the oil flow rate is maintained at a level sufficient to ensure the continued satisfactory functioning of the seal and avoidance of the seal overheating.
Such hydraulic seals are very effective in preventing the flow of gas from a region of high gas pressure to a region of low gas pressure. However, they can give rise to problems if one of the shafts suffers a major failure and fractures.
The main shafts of a gas turbine engine interconnect the rotary air compression portions of the engine with its rotary turbine portions. Thus the rotary turbine portions drive the rotary air compression portions via the shafts. If one of the shafts should fracture, the load driven by its turbine portion is suddenly removed, thereby resulting in a rapid increase in its rotational speed. This increase is so rapid that if measures are not taken to slow it down, it will explode. Clearly such explosive turbine failure is not acceptable and so provisions are normally made to ensure that turbine braking takes place in the event of a shaft failure so that the critical explosive speed is not reached.
If a shaft fractures, its rearward part rapidly translates in a rearward direction until the rotor and stator parts of the turbine that it carries engage each other. The turbine is so designed that this results in a rapid slowing down of the turbine, thereby preventing its explosive failure. However, hydraulic seals of the type described above can have an adverse effect upon the effective operation of this mechanism. This is due to the fin and oil-containing parts of the seal engaging each other when the shaft moves rearward, thereby inhibiting further rearward shaft movement. As a result the turbine does not slow down to a speed sufficiently low to prevent its catastrophic failure.